In the grinding of rags and the like for the preparation of a furnish for roofing felt, for example, it is necessary to determine when the stock has arrived at desired condition. Threads that are too long are difficult to handle and result in an inferior product. Fibers that are too short drain slowly and interfere with efficient manufacturing. Presently, the only available method for determining quality is to manually examine a handful of stock. Not only is such procedure time consuming, but it is also most imprecise depending on the skill and experience of the operators involved. As a result, product quality is impaired, productivity is reduced and operating cost increased.
In recent years, continuous digesters have replaced batch digesters, i.e. wood chips and chemicals are continuously introduced to, and pulp is delivered from, a complex system of vessels, piping, etc. In many continuous pulp mills, a problem is encountered inasmuch as several species of wood are utilized and it is essential to segregate resulting pulp according to specie, i.e. a pulp made from softwood must be segregated from pulp made from sawdust, etc.
The problem is that, as it emanates from the pipeline, all pulps are virtually indistinguishable. There is, however, one factor which quantitatively identifies virtually all species, namely fiber length. By continuously monitoring fiber length of pulp as it passes through the pipeline, a change from one specie to another becomes immediately evident, positively precluding any possibility of a customer receiving incorrect pulp.
Thus, an instrument which will automatically monitor fiber length of refined paper stock, or of unrefined paper pulp, while in slurry form, at operating consistencies and while in the paper making system will obviously be most useful.
The principal object of this invention is to provide such an apparatus and method so that a signal is generated to inform the operator when a pulp interface passes, or that stock is refined to the correct fiber length and quality.
Use can be made of the signal to automatically control the refining effect of refiners to optimize, and maximize, uniformity.
It has, heretofore been proposed in U.S. Pat. No. 1,580,166 to Reid of Apr. 13, 1926 to provide a laboratory type device with a screen having elongated, narrow, slots, a tank entirely separate from a stock pipeline system and a vacuum mechanism for drawing stock through the screen. The slots are about one hundreth of an inch in width so that a dilute slurry is used and the stock is treated as discrete fibers, rather than as a fiber aggregate. The Reid device would not be capable of instantaneous "read out" in "on-line" operation since one would have to count, or weigh, the fibers which do not pass the screen.
In U.S. Pat. No. 2,973,000 to Pearson of Feb. 28, 1961, a sharp edged orifice plate is used to determine consistency of diluted stock in a laboratory type device. All of the fibers pass through the orifice without regard to length.
In U.S. Pat. No. 3,846,231 to Crosby et al of Nov. 5, 1974 the stock is drawn across a perforated partition to measure freeness just as in my two patents on freeness testers cited thereagainst namely U.S. Pat. No. 3,538,749 of October 1970 and U.S. Pat. No. 3,186,215 of June 1965 both to Danforth. No claim, or teaching, is made in these patents for measuring or monitoring fiber length.
A prior patent which more closely relates to measuring fiber length is U.S. Pat. No. 3,873,416 to Forgacs of Mar. 25, 1975 but the device requires the stock to be substantially diluted rather than at operating consistency in the paper making system. The Forgacs device works on a continuous flowing stream principle and not on a sampling sequence and makes use of a fractionating screen which is vertically oriented and vibrates.
As far as I am aware, none of the devices of the above patents are available in the trade and no instantaneous, automatic, "read-out" of fiber length, has been developed, except as disclosed herein.
There are no other references in the prior art, known to me, disclosing a device to monitor fiber length directly in the production system.